1. Field of the Invention
This invention relates to the field of video special effects, and more particularly, to a method and means for creating and establishing priority among a plurality of masks affecting the nature of a video picture.
2. Brief Description of the Prior Art
Masking techniques are known in which certain picture segments, known as masked areas, are modified or enhanced in accordance with a specified condition referred to herein as a conversion formula. Each conversion formula is operative within the boundaries of its masked area, and when two masked areas overlap, the area of the picture in which overlapping occurs is affected by the conversion formula of both segments combined. This combination of conversion formulas in the area of overlapping segments may be used to advantage to obtain interesting patterns or effects. It can, however, be deleterious to the overall enhancement scheme, especially when the enhancement effect is that of coloring previously black and white pictures or modifying the coloring of a previously colored picture.
The degrading effects of the combining of conversion formulas can be appreciated from the fact that, due to the light-emitting nature of the phosphor on the face of a picture tube, light emitted from the same area of the picture tube produced by video signals representing two different colors mixes according to a color additive process. For example, if two masks overlap and in the overlapped area, one displays a green color and the other displays a red color in a prescribed proportion to the green, then when the two colors are visually mixed on the face of a color picture tube or screen, the resultant color would be yellow. Since neither color substantially dominates the other, and since a yellow color is strikingly different than either red or green, if two objects are close together having red and green coloring, respectively, and the picture segments in which the respective color conversion formulas are operative overlap, the undesirable yellow hue will result.
In certain systems, such as those that use a computerized "color tablet" to "paint" a group of pixels in one solid color, the problem is avoided by not allowing any picture segments, or masks, to overlap. Accordingly, when one object moves into another, the colorizer technician reshapes the area of pixels and effectively manually prioritizes the colors of all areas of the picture. Alternatively, the computer in such a system can store characteristics of each pixel and make educated judgments as to changing positions of object shapes so as to "electronically" follow a moving object. In either case, however, when the system is confronted with one object moving into the other, or encounters the close proximity of two differently colored objects, a system operating in the manner just described becomes extremely slow in the coloring process, because the colorizer technician must carefully draw a large number of masked areas on the color tablet in precise relationship to the outlines of each object.
In prior art systems which operate on the basis of establishing a color conversion formula in which different colors are chosen on the basis of the luminance level of the picture within a prescribed area, a similar problem is encountered but of considerably less degree. A system which uses gray levels of the luminance signal for establishing colorization formulas will, nevertheless, often encounter objects that are closely positioned and/or move relative to other objects, such that the segments or masks, within which color conversion formulas are active, overlap. The net colorization formula in the area of mask overlap is thus some combination of the colorization formulas contributing in that area of overlap.
One such system using the gray level for establishing color conversion formulas can be found in my U.S. Pat. No. 4,149,185 issued Apr. 10, 1979, the disclosure of which is incorporated herein by reference in its entirety. In accordance with the system described in U.S. Pat. No. 4,149,185, the outputs of a plurality of multipliers are summed together to produce the picture components Y, R-Y, and B-Y, such components being combined to form a composite color video signal or RGB video signal for display on a monitor. Each of the three components has its separately summed multipliers, and each multiplier has as its two inputs a color conversion formula and a mask signal which enables the color conversion formula to be active only within a prescribed masked area of the picture.
Each multiplier receives its mask signal input from a mask generator, and each mask generator is operated totally independent of other mask generators. Accordingly, although each separate color conversion formula is affected by its associated multiplier for a particular designated area of the picture, nevertheless the independence of the mask generators and the standard summation technique used in summing the outputs of the multipliers result in a weighted colorization formula for picture elements of the same video level in any area where any two or more masks overlap, the weight of each color in the area of overlap being dependent upon the strength of that color according to the color conversion formula it represents.
As set forth in my prior U.S. Pat. No. 4,149,185, one of the most important advantages over the "pixellated" colorization technique is that, since gray levels are assigned specific colors, the movement of a colored object, or its proximity to other colored objects, does not result in any deleterious effects for so long as each gray level is to maintain its assigned color. When two masks overlap, however, there are occasions when problems arise if the same video gray level is assigned different colors for the two overlapping masks. While this is less of a problem with systems in accordance with my prior U.S. Pat. No. 4,149,185 than that of systems using colored pixel groups, there is nevertheless an established need for eliminating the problem of color-mixing in areas of the video picture at which two or more masks overlap. The present invention fulfills this need.